UHMWPE is difficult to process because the resin does not flow when melted. Berins, M. L., ed., Plastics Engineering Handbook of the Society of the Plastic Industry, Chapman & Hall, New York City, N.Y. (1991), p. 52. Consequently, UHMWPE is processed by sintering, compression molding, ram extrusion, or gel processing. Sintering is a process where resins are agglomerated by solid-state diffusion. Heat and pressure are usually essential. Compression molding is a process where resins are shaped between the faces of a mold by heat and pressure. Ram extrusion is a process where resins are shaped by forcing it through a die. The force is provided by a ram. U.S. Pat. Nos. 5,234,652 and 5,399,308 disclose a dry extrusion process for a mixture consisting of resins and lubricant. Gel processing is a process where resins are formed into gels for subsequent processing. The gel is a dilute solution or suspension of resin in a solvent, e.g., an extractable solvent (or oil or plasticizer).
UHMWPE is gel spun into fibers. In gel spinning, the primary mechanism of solidification is the gelling of the polymer solution by cooling to form a gel filament consisting of precipitated polymer and solvent. Solvent removal is accomplished following solidification by washing in a liquid bath. This process is also used to form microporous films. See U.S. Pat. Nos. 4,588,633 and 5,248,461. In U.S. Pat. No. 4,588,633, the gel solution consists of 2-4% by weight of UHMWPE. See Examples 1-12. In U.S. Pat. No. 5,248,461, the gel solution consists of up to 20% by weight of UHMWPE. See Examples 1-20. These membranes are useful as, among other things, separators for electrochemical cells. See U.S. Pat. No. 4,588,633, column 4, lines 30-36 and U.S. Pat. No. 5,248,461, column 4, lines 57-60.
Another variant of gel processing is gel processing with a filler. For example, see U.S. Pat. Nos. 3,351,495, 4,833,172, and 5,948,557. Generally, UHMWPE, a processing oil (or plasticizer), and a filler are mixed in an extruder and subsequently made into microporous sheets. In U.S. Pat. No. 3,351,495, the solution consisted of up to 20% by volume of UHMWPE. See Examples 6, 8, 18, 19, 20, and 21. The microporous membranes formed were used as separators for batteries. See column 1, lines 24-33. In U.S. Pat. Nos. 4,833,172 and 5,948,557, a calcium/zinc stearate lubricant, PETRAC® CZ-81, is added to the solution. See Tables 1 and 2, respectively. The microporous membranes formed were used as labels, diffusion membranes, and separators. See U.S. Pat. No. 5,948,557, column 1, lines 27-41.
There is a need to improve the processability of UHMWPE.